The subject matter herein relates generally to mode size converters that change a modal profile of propagating light and optical devices including the mode size converters.
Recently, more and more industries have begun to use optical devices and, in particular, optical devices developed through silicon photonics. Such optical devices include photonic integrated circuits (PICs), which may be used for various applications in optical communications, instrumentation, and signal-processing fields. A PIC may include submicron waveguides to interconnect various on-chip components, such as optical switches, couplers, routers, splitters, multiplexers/demultiplexers, modulators, amplifiers, wavelength converters, optical-to-electrical signal converters, and electrical-to-optical signal converters. One advantage that PICs have is the potential for large-scale manufacturing and integration through known semiconductor fabrication techniques, such as complementary metal-oxide-semiconductor (CMOS).
A PIC may be optically coupled to an external optical fiber such that the PIC receives light from the optical fiber and/or directs light into the optical fiber. However, it can be challenging to optically couple the optical fiber and the PIC in a reliable and efficient manner. For instance, the optical fiber has a cross-sectional area that is much larger than the cross-sectional area of the submicron waveguide of the PIC. Consequently, light propagating within the optical fiber will have a much larger modal profile than the modal profile of the light in the waveguides of the PIC. When the light transitions between the optical fiber and the PIC, the modal profile of the light must change in size (a process referred to as mode conversion) without significant losses.
One known mode size converter (or spot size converter) includes an overlay (or cladding) waveguide and a silicon waveguide that is embedded within the overlay waveguide. The silicon waveguide has an inverted taper geometry in which a tip of the silicon waveguide is positioned proximate to an edge of the overlay waveguide. As the silicon waveguide extends from the tip, a width of the silicon waveguide adiabatically widens to a final cross-sectional area that is capable of supporting a propagating mode. Light from the optical fiber enters through the edge of the overlay waveguide and is coupled evanescently to the silicon waveguide. The light becomes progressively more confined as the silicon waveguide widens to the single-mode strip waveguide. Accordingly, the modal profile of the light from the optical fiber is reduced to a size that is suitable for propagating through the silicon waveguide.
Although such mode size converters can effectively reduce the modal profile, the mode size converters may have some challenges or drawbacks. For example, the mode size converter may have a coupling efficiency that is insufficient, may have a low tolerance for alignment, and/or may be commercially impractical to manufacture.
Accordingly, there is a need for a mode size converter that has a sufficient coupling efficiency, a high tolerance for alignment, and/or is not cost prohibitive to manufacture.